Tests for Ions

Tests for Ions

Tests for Cations (Positive Ions)

  • Flame Tests: Some metals produce a characteristic colour when heated. Sodium gives a yellow flame, potassium a lilac flame, calcium an orange-red flame, and copper a blue-green flame.
  • Sodium Hydroxide Tests: Many transition metals form coloured precipitates when reacted with sodium hydroxide. Iron(II) ions form a green precipitate, while iron(III) ions form a brown precipitate.
  • Ammonium Ions (NH4+): When heated with sodium hydroxide, ammonium compounds produce ammonia gas which can be detected by its pungent smell, or by turning damp red litmus paper blue.

Tests for Anions (Negative Ions)

  • Carbonate Ions (CO3^2-): When reacted with an acid, carbonates produce carbon dioxide gas which effervesces (fizzes). The gas can be tested with limewater, which it turns cloudy.
  • Halide Ions (Cl^-, Br^-, I^-): When halides are reacted with silver nitrate, they form a precipitate of silver halide. The colour of the precipitate will indicate the presence of chloride (white), bromide (cream), or iodide (yellow).
  • Sulphate Ions (SO4^2-): A white precipitate of barium sulphate is formed when sulphate ions are reacted with barium chloride in the presence of dilute hydrochloric acid.

Tests for Gases

  • Hydrogen: A lit wooden splint produces a “pop” sound in a test tube of hydrogen gas due to its rapid combustion in air.
  • Oxygen: A glowing wooden splint rekindles in oxygen due to its ability to support combustion.
  • Carbon Dioxide: Turns limewater cloudy due to the formation of calcium carbonate.
  • Chlorine: Bleaches damp litmus paper, leaving it white.

This test revision content can be utilized for better understanding or to prepare for assessment tasks. Always remember to identify the ions present based on the unique reactions and colours observed.